I run 18" MTH Premier passenger cars exclusively on my layout, which has O72 curves and some that are larger. They look terrific on my layout and are close enough to scale for my purposes. BTW, Lionel made some very nice, highly-detailed 18" passenger cars for several years before migrating to the 21" cars a few years ago.
I bought a number of 21" K-Line passenger car sets in the early 2000s and one GGD passenger car set more recently. They are all beautiful, but I decided they were simply too large for my layout and am in the process of selling them all.
Pat